1. Technical Field
This invention relates to a package for a microcircuit, such as an integrated or hybrid circuit, and a method of hermetically sealing the microcircuit package.
2. Background Art
Packaging and sealing techniques for integrated and hybrid circuits are well-known. Generally, an integrated circuit (I.C.) refers to a single chip (die) device which requires only a small cavity or space for packaging the chip. A hybrid circuit generally refers to a multi-component assembly which requires a larger cavity for packaging. In this specification, the term microcircuit will refer both to I.C.s and to hybrid circuits.
Integrated and hybrid circuits must be enclosed or packaged for most commercial applications in order to make handling them easier and to protect the circuit. It is common to assemble these circuits in small packages known as flat packs, because of their essentially flat shape and planar leads. Other commonly used packages are plug-in packages with leads at right angles through the package body and dual in-line packages with two rows of parallel leads at right angles to the package body. The leads of a dual in-line package are outside of the package body instead of through the package body as with plug-in packages. The packages are usually sealed by a lid. The circuits in these packages must be hermetically sealed to isolate the circuit from the atmosphere, dirt, moisture, and other contamination which could destroy the circuit or affect its operation. Obviously it is desirable to be able to produce hermetic packages in high volumes at low cost and be able to seal them hermetically in high volumes at low cost, while still maintaining an effectively protected microcircuit.
The four basic types of seals commonly used for hermetically sealing a microcircuit package are a weld seal, a solder seal, a frit seal, and a plastic seal. A welded seal is generally the most expensive and the slowest to make, however, the hermeticity and yield achieved with a welded seal is the best. Plastic seals are the least expensive, but their hermeticity is poor.
Welded seals may be made by any one of a number of conventional methods. Welded seals may be made also with energy forms such as lasers or electron beams. Seam welding usually is done by delivering welding current through two opposed rollers resting on opposite sides of a lid for a microcircuit packages. Normally this method is used with a stepped lid on a metal package. Packages of ceramic, glass, or other non-metallic materials can be used with the addition of a metal seal ring of sufficient thickness to protect the package from the thermal stress of welding. Seam welding tends to be a very slow process, requires a flat sealing surface, and is relatively expensive.
Resistance welding is an electrical weld where the entire perimeter of the package/lid is sealed simultaneously and the current flows through both the package and lid. A triangular projection on either the package or lid is normally used to develop a point of high electrical resistance to aid getting a uniform weld all the way around the sealing surface. This welding method requires expensive equipment and packages which must be specifically tooled. Very large power sources are needed when using alternating current for resistance welding.
Cold welding requires that the package and lid have a soft malleable material on their mating surfaces. The package and lid are forced together under high pressure so that the two mating surfaces diffuse into each other. This method requires special package designs and expensive raw materials and equipment. The package produced is not very durable.
Soldered seals also may be produced by any one of several well-known methods. The package and lid with solder between them is heated to form a hermetic seal. Generally, the whole assembly is held in alignment with a spring clip or lies in a mold with the weight of the components or some additional weight forcing together the surfaces to be soldered. The solder material used for this type of seal usually includes gold and both of the sealing surfaces are usually gold-plated. Because of the high price of gold, this method is very costly. Flat sealing surfaces are also required.
Perimeter soldering is done by heating the lid while heat sinking the package, or only heating the perimeter of the lid while cooling the center of the package. With this method, gaseous pressure in the package must be controlled carefully so that the solder does not get blown out during heating or sucked into the package while cooling. As with other soldering methods, this method requires a flat seaing surface, usually requires gold on the sealing surface, and is a relatively slow process.
So-called "frit" seals require a low temperature glass instead of solder for sealing microcircuit packages. This method does not require gold plated sealing surfaces. Flat sealing surfaces are not as critical to this method as they are in the weld and solder sealing methods. However, the microcircuit package is subject to higher and thus potentially more damaging heat than with other methods. The hermeticity of the seal is not as effective as in other sealing methods.
"Cerdip" is a particular package whose name has been taken for a sealing method. "Cer" refers to ceramic and "dip" to Dual-In-Line, a package configuration. This method requires a lead frame placed between two pieces of ceramic which have been glazed with frit on the sides facing the lead frame. The package is processed to melt the frit and thus seal the package.
Hot cap sealing is a frit-related sealing method. A perimeter of frit is placed on the package and sometimes the lid. The lid is heated above the melting point of the frit and forced onto the package, thus melting the frit on the package and forming a hermetic seal.
Epoxy sealing is the name generally given to sealing a lid to a package with plastic medium, sometimes in the form of a preform, much the same as solder sealing. Heat is not generally required to melt the preform. Some package styles permit sealing with poured or dispensed epoxy. Although this sealing method is relatively inexpensive and does not require special equipment or especially flat sealing surfaces, the quality of hermeticity of the seal is poor.
Conformal coating a microcircuit to seal it is well-known. The microcircuit is usually immersed or dipped into fluid plastic. The coating will be either self-curing or require a heat cure. In most cases, the conformal coating obviates the need for a package for the microcircuit. As an alternative to dipping the microcircuit into fluid plastic, a fluid bed be may be used to keep epoxy powder suspended in a stream of air or inert gas. The microcircuit package is dipped into the powder after it is heated so the particles of epoxy melt and thus seal the microcircuit. The microcircuit package also may be sprayed with liquid form plastic or epoxy powder instead of dipping it. This sealing method is relatively inexpensive, but it provides a seal of poor hermeticity and has poor dimensional stability.
Plastic molded packaging requires the microcircuit to be bonded in place on a lead frame and the assembly then placed in a mold and encapsulated in plastic. Sometimes a drop of epoxy or similar material is put on the microcircuit to protect it and the wire bonds from the movement of the plastic encapsulant during the molding. Although this process is very inexpensive, the hermeticity of the seal is very poor.
Both the I.C. and the hybrid manufacturer want good, low cost protection for circuits. This is far more difficult to achieve, however, for hybrid circuits. Since the hybrid circuit is generally much larger than the I.C., it is more difficult to package. The larger the dimensions, the more difficult it is to achieve flatness, which is required for the prior art methods which give high hermeticity. The hybrid often contains may different components other than the single chip of the I.C. and thus its processing often has limitations not found with I.C.s. Some hybrids contain components such as electrolytic capacitors or extremely precise resistors and therefor can not withstand a "hot" method of sealing.
Seemingly, the I.C. is far less demanding of its seal process, since is smaller than a hybrid and there is only one chip to seal. Since I.C.s are produced in such large volumes, however, any slight improvement in their sealing can offer substantial cost savings.